The operation of a commercially available nebuliser system (Medic-Aid Ltd) is reviewed and the efficiency with which it produces an aerosol assessed. Defining the efficiency of nebulisation E as the fraction of the original mass of solution released as an aerosol it is found that the internal surface area, mass of solution used, the surface tension of the solution and the angle of tilt are important factors in determining E. Reducing the internal surface area of the nebuliser by means of Perspex inserts significantly increases E for 3 g of water from 49% for the unmodified system to 67% for the modified nebuliser (P less than 0.01). E increases with the mass of solution used but only exceeds 60% when 4.5 g water are used. Decreasing the surface tension of the solution from 7.2 x 10(-3) N m-1 (water) to 3.7 x 10(-3) N m-1 and 3.1 x 10(-3) N m-1 (using two different concentrations of a detergent in water) significantly increases E for 3 g solution from 49% to 65% and 69% respectively (P less than 0.05). Operating the nebuliser at a tilt also increases E. The measurements emphasise the importance of reducing the internal surface area of this type of nebuliser, using an adequate volume of drug solution (at least 4 ml is suggested) and operating the nebuliser at an angle to the vertical (20 degrees suggested) in order to maximise E. The surface tension of the drug solution is a further important determinant of E.